Optical Bypass — Build Documentation V1.0

01

Overview & Features

This is a Voodoo^®-style true-bypass switching board in a very small form factor — about the size of the switch itself. It does the job of a standard 3PDT click-stomp footswitch but uses an optocoupler in the audio path and a standard DPDT latching switch for the mechanical action. The result is a bypass with the same routing behaviour you expect, but without the loud contact-clack you get from a hard-switching 3PDT.

It's intended as a drop-in replacement: wire the IN and OUT pads to your input and output jacks, the TO and FRM pads to your effect's input and output, and the +9V/GND pads to your power supply. The board fits inside almost any enclosure — even very small ones where a 3PDT wouldn't physically go.

Silent switching

The optocoupler ramps the audio path on and off over a few milliseconds — no sharp transient, no audible click.

Standard DPDT switch

Uses a regular DPDT latching footswitch (Alpha, Tayda, etc.) — much cheaper and more reliable than a 3PDT.

True bypass when off

In bypass mode, the input is wired directly to the output through the mechanical switch — no buffer in your signal path.

Status LED included

The on-board indicator LED shares current with the optocoupler's internal LED — when the LED lights, the effect is engaged.

Tiny footprint

Just six pads on the perimeter (IN, OUT, TO, FRM, +9V, GND) — fits anywhere there's room for the switch and an LED.

Tunable LED brightness

The CLR resistor sets the LED current — pick a value to match the colour and brightness you want.

Populated Optical Bypass PCB — Populated prototype board (CPC1017N optocoupler).

02

Circuit Theory

The circuit splits into two functional parts: an audio path (the right-hand side of the schematic) and an LED-drive control path (the left-hand side). Both halves are operated by a single DPDT latching switch — one pole runs the audio, the other runs the LED.

Audio path

The IN pad connects directly to two places at once: the optocoupler's output transistor (collector) and pin 1 of switch pole _A. The optocoupler's other output pin (emitter) goes to the TO pad — “to your effect's input”. The FRM pad — “from your effect's output” — connects to pin 3 of switch pole _A. Pin 2 of switch pole _A is the OUT pad.

So switch pole _A picks which signal reaches OUT: in one position, the IN signal goes straight to OUT for true bypass; in the other position, the FRM signal (the effect's processed output) goes to OUT. Independently of that, the optocoupler decides whether IN reaches the effect's input (TO pad) — when the LED is on, the optocoupler's photo-MOSFET conducts and IN is connected to TO; when the LED is off, the photo-MOSFET is open-circuit and the effect's input is silent.

LED control path

The +9V rail feeds the current-limit resistor CLR (4k7), then the indicator LED D1, then the optocoupler's internal LED (anode at OC1 pin 1, cathode at pin 2). The cathode of the optocoupler LED is the “return” node — to light the LED, this node has to connect to GND. Switch pole _B is what connects it: in the engaged position, _B closes the cathode-to-GND path and current flows through both LEDs; in the bypass position, _B opens that path and both LEDs go dark.

C1 (100 nF) sits across the optocoupler-LED-cathode-to-GND node and provides high-frequency bypass — it smooths the LED-current edge during switch contact bounce and helps slow the rise/fall of optocoupler activation. This is the key to the click-free behaviour, explained in detail in section 03.

State summary

Switch position	Audio path (pole A)	LED state (pole B)	Optocoupler	Result
Bypass	IN → OUT direct	LED off	Open	Dry signal at OUT, effect silent
Engaged	FRM → OUT	LED lit	Conducting	IN → effect → OUT

Optical Bypass schematic — Full schematic — Optical Bypass V1.0.

Why this is true-bypass. When the switch is in bypass mode, the IN signal travels through one mechanical switch contact (pole _A, position 1↔2) directly to OUT. There is no active component in the dry path — no buffer, no JFET, no opamp. The optocoupler sits on a side branch (IN → opto → TO) which is open in bypass, so it doesn't load the dry signal in any meaningful way.

03

How Click-Free Switching Works

A traditional 3PDT footswitch makes its audible “click” for two reasons. The obvious one is the mechanical detent of the switch itself. The less obvious — and more problematic — one is the electrical transient created when contacts close in microseconds and instantly drop or raise the DC potential of the audio line. That transient appears as an audible “pop” superimposed on the signal, and depending on input capacitance and gain staging it can be quite loud.

This board addresses the electrical click only — the mechanical switch detent is still there, just like any latching footswitch. But the audio routing happens through a photo-MOSFET whose state changes gradually rather than instantly:

The transition timeline

1. Switch contact begins to close. The mechanical contacts bounce briefly — a raw connection through them would create a burst of microsecond on/off transients.

2. Contact bounce hits the LED line, but C1 absorbs it. C1 (100 nF) at the optocoupler-LED cathode node smooths the noisy current ramp into a clean, monotonic rise.

3. The two LEDs (indicator + optocoupler internal) start conducting. Because they're in series, both light up simultaneously. The indicator LED's brightness ramps up visibly, and the optocoupler's internal infrared LED illuminates its photodiode-driver inside the same package.

4. The photo-MOSFET turns on over ~1–3 ms. This is the published turn-on time for the CPC1017N — far slower than a mechanical contact, but still well below the threshold of audible clicking (typical click thresholds are around 50 µs and shorter for a sharp transient). The audio path opens and closes as a smooth ramp, not a step.

5. Steady state. Once on, the photo-MOSFET behaves as a low-resistance bidirectional switch (typically 10–25 Ω on-state) — totally transparent to line-level audio. When the switch flips back, the same process happens in reverse: LEDs dim, photo-MOSFET ramps off, audio path closes silently.

Why the CPC1017 (and not just any optocoupler). The CPC1017N is specifically a photo-MOSFET output device — a true bidirectional analogue switch. Most cheap optocouplers (4N25, TLP181, PC817) have a phototransistor output that conducts in only one direction and distorts AC signals. A photo-MOSFET passes audio cleanly in both directions with no rectification.

Why the indicator LED is in series

Wiring the on-board indicator LED in series with the optocoupler's internal LED gives you a free visual confirmation that the optocoupler is doing its job. If the indicator lights up but there's no audio coming through TO, you know the LED side is fine and the fault is elsewhere (probably a missing TO–effect connection). It also halves the parts count compared to driving two LEDs in parallel from separate resistors.

Series LED forward voltage budget. Two series LEDs drop roughly 1.2 V (opto IR LED) + 2.0–3.2 V (indicator LED) = about 3–4 V total. From a 9 V supply through a 4k7 resistor, that gives roughly (9 − 3.5) / 4700 ≈ 1.2 mA. That's plenty for the optocoupler — it specifies 1 mA forward current as the typical drive — and gives a comfortable indicator glow for a standard 3 mm red or green LED. Brighter colours (white, blue, “super-bright”) want a smaller resistor — try 3k3 or 2k7. Dim or low-current LEDs want a larger one.

04

Bill of Materials

Tiny parts count — six components plus the switch. The single resistor is the LED current limiter; tune it to your indicator LED's preferred current.

Ref	Qty	Value	Colour code	Notes
Resistors — 1/4 W metal film, 1%
CLR	1	4.7 kΩ	Yellow · Violet · Black \| Brown · Brown	LED current-limit. Sets brightness for both indicator LED and optocoupler internal LED. 3k3 brighter 10k dimmer — pick to taste.
Capacitor
C1	1	100 nF	—	MLCC ceramic, 2.5 mm pitch. Smooths LED-current edge during switch contact bounce.
Diode
D1	1	LED 3 mm or 5 mm	—	Indicator LED. Any colour. Long lead (anode) goes into the square pad. 3 mm fits the silkscreen exactly; 5 mm works if oriented carefully.
Optocoupler
OC1	1	CPC1017N	—	Photo-MOSFET solid-state relay, SOIC-4 (SMD). Must be a photo-MOSFET / SSR type — bidirectional output is essential for audio. Through-hole DIP-4 footprint also accepts compatible parts. CPC1017NTR LCA110 AQY212
Switch
S1	1	DPDT latching	—	Standard PCB-mount or panel-mount latching footswitch. Alpha or Tayda parts both fit. Mounts on the opposite side of the board from the other components.
Hardware
—	—	Hookup wire	—	Stranded 22–24 AWG for the six perimeter pads (IN, OUT, TO, FRM, +9V, GND).

Do not substitute a phototransistor optocoupler. Generic four-pin optocouplers like 4N25, 4N35, PC817, or TLP181 use a phototransistor output that only conducts current in one direction. They will half-wave-rectify audio, producing severe distortion and DC offset on the output. The CPC1017 / LCA110 / AQY212 family is specifically photo-MOSFET / solid-state-relay parts — they pass AC cleanly in both directions.

About the CLR resistor. Start with 4k7. If the indicator LED looks too dim for your taste, drop to 3k3 or 2k7. If it's blinding, raise to 10k. The optocoupler threshold is around 0.5 mA, so anything from a couple of kΩ up to about 22k will keep the optocoupler firmly on; above that, the audio path may not fully open.

05

Build Guide

Six components, ten minutes. Mount the small parts first, then the switch from the back side of the board, finally the LED.

PCB silkscreen — top view — PCB silkscreen — components-up view.

Populated PCB photo — Reference: populated board.

Optocoupler (OC1)

Solder the optocoupler first — it's the smallest part, easier to place when nothing else is in the way. Align the marker dot on the package with the circle on the silkscreen (this marks pin 1). For SMD CPC1017N: tack one corner pad first, check alignment, then solder the rest.

Capacitor (C1)

Install C1 (100 nF MLCC). Ceramic capacitors are not polarised — orientation does not matter.

Resistor (CLR)

Install CLR — 4.7 kΩ to start (yellow · violet · black · brown · brown for 1% five-band). Resistors aren't polarised, but if your board layout has a recommended orientation it's a nice touch to follow it for tidy reading later.

Switch (S1) — from the back

The DPDT latching switch mounts on the back of the board (opposite side from the resistor and cap). Insert the switch lugs through the board, push it flush, and solder on the component side. This puts the switch shaft pointing down through the enclosure.

Indicator LED (D1)

Solder the LED last — installation height should match what your enclosure needs. The long leg goes into the square pad (this is the anode, connected to the resistor side). Get this right: a reversed LED won't damage anything, but it will also not light up.

Wire up

Solder hookup wires to the six perimeter pads — IN, OUT, TO, FRM, +9V, GND — long enough to reach the input/output jacks, the host effect's IN/OUT, and your power tap. See section 06 for the wiring diagram.

Test before final installation

Apply power. Toggle the switch — the LED should come on in the engaged position and go dark in the bypass position. With audio at IN, you should hear dry signal at OUT in bypass and the host effect's wet signal in engaged mode. If the LED never lights, check the LED orientation. If audio gets through in bypass but not in engaged mode, check the TO/FRM connections to your host effect.

06

Wiring

The board has six external pads. The wiring diagram below shows how to hook them up to a pair of mono input/output jacks and a host effect.

Pad	Goes to	Purpose
IN	Input jack tip	Dry signal in
OUT	Output jack tip	Switched signal out
TO	Effect input	Sends signal to your effect when engaged
FRM	Effect output	Brings the wet signal back to OUT
+9V	Power supply +	9V from your effect or pedalboard supply
GND	Power supply −, jack sleeves	Common ground

Wiring diagram showing connections to input/output jacks and the host effect — Reference wiring — IN/OUT to mono jacks, TO/FRM to the host effect's input and output.

Sharing power with the host effect. The +9V and GND pads can simply tap off the same supply that powers the host effect — you don't need a separate power feed. Make sure all grounds (jack sleeves, effect ground, supply ground) meet at one star point to avoid hum loops.

07

Setup & Troubleshooting

If the LED is too dim or too bright

Change CLR. Drop the value to brighten (3.3 kΩ, 2.7 kΩ, 1 kΩ if your LED is very dim — but don't go below 1 kΩ or you'll over-drive the optocoupler input). Raise the value to dim (10 kΩ, 22 kΩ — above 22 kΩ the optocoupler may not fully turn on).

If the audio doesn't switch (always bypassed)

Likely the indicator LED works but the optocoupler isn't conducting. Check that the optocoupler is the correct type — a phototransistor opto won't work here. Also verify TO is going to the effect's input and FRM is coming from the effect's output; swapping them silently disables the engaged mode.

If you hear residual click on switching

A tiny click can sometimes be heard if the host effect itself has a high-impedance input with no DC reference, leaving its input floating in bypass mode and pulling charge through the photo-MOSFET when it opens. The fix is on the host effect side — add a 1 MΩ resistor from the effect's input to ground to give it a defined DC level at all times.

If both LEDs stay dark

Re-check: indicator LED orientation (long leg in square pad), CLR resistor value, +9V reaching the board, and DPDT switch wiring. With the switch in the engaged position, you should be able to measure roughly 0 V at the optocoupler's pin 2 (cathode of internal LED) with respect to GND. If pin 2 reads near 9 V, the switch is on the wrong pole or the cathode-to-GND connection is broken.

Replace your noisy 3PDT. Once it's wired and working, this little board fits almost anywhere — perfect for cramped enclosures, hammered-on-the-bench prototypes, and any pedal where the loud click of a 3PDT footswitch is annoying through a quiet rig.

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Disclaimer & Licence

PCBs purchased from TH Custom Effects are intended for DIY and non-commercial use only. Redistribution of PCBs and artwork from this document is not permitted. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.